The proposed experiments seek to characterize the capacity of the visual system for transmitting sensory information. The approach is both at a high level, by studying visual attention, and at a low level, by measuring intrinsic noise. The work on attention, Aim 1, addresses the "span of attention" conjecture that attention is mediated by a general-purpose processor with a data capacity of only 45 bits (Verghese and Pelli, 1991). This will be tested by measuring the data capacity for identifying words and reading. The data capacity of the display will be restricted by blur and added noise, to find the lowest data capacity that allows unimpeded reading. The results will either disprove the span-of-attention conjecture, by finding a higher data capacity, or confirm it, showing that reading rate is bounded by the fixed data capacity of visual attention. The intrinsic noise measurements of Aims 2 and 3 borrow a technique from electrical engineering. Noise will be presented as a visual stimulus, and measured effects of that noise will be used to infer the characteristics of the intrinsic noise, expressed as an equivalent input noise. Aim 2 is a psychophysical survey of the spatiotemporal spectrum of the equivalent noise as a function of luminance and retinal eccentricity for rods and cones. Aim 3 is a physiological measurements of the equivalent noise in cat ganglion cells. The results of Aims 2 and 3 will provide a thorough description of the properties of the intrinsic noise, which may then be incorporated into models of visual sensitivity.